Mixed drug use and mental ill-health – defence considerations. By Allison Summers K.C. & Giles Fleming.

It is a scenario with which most defence practitioners will be all too familiar. A defendant who was suffering from obvious mental ill-health at the time of the offending; but who also has a history of drug abuse. What effect, if any, will their mental health have on the case?

Intent

Possibly the simplest question to address first, for crimes of “specific intent”, is whether the defendant did or did not have the requisite intention. For example, for an offence of murder, did they in fact intend to kill or cause really serious harm? If they did not, then they will have a defence to specific intent crimes. This applies regardless of the reason for that lack of intent – even if the only reason they lacked intent was because of voluntary intoxication through drugs. It’s therefore not necessary here to be able to prove whether a particular effect is the result of the drug use, or the mental ill-health, or any other factor.

There is some difficulty in precisely identifying specific intent offences, in the wake of R v Heard, which suggests that it is not necessarily as straightforward as saying (as had been thought before Heard), that offences that can only be committed with a particular intention are specific-intent offences, whereas those which can be committed recklessly are basic-intent offences. That being said, it seems to remain uncontroversial that key offences, including murder, are crimes of specific intent, so can take advantage of this rule.

The greater difficulty is often the evidential one: will a jury really believe that the defendant who strangled or repeatedly stabbed the victim did not intend to cause really serious harm, however intoxicated and/or unwell they were? The rarity of that evidential position being established is perhaps why the uncertainties created by Heard almost 20 years ago have not yet been resolved by further cases in the Court of Appeal.

Cause or effect

If the defendant is not going to be able to persuade the jury that they lacked (or may have lacked) the requisite intent, it’s likely to be crucial to establish whether the mental ill-health was caused, or substantially exacerbated, by the drug use. Whilst it often may be, this is by no means a given.

• Sometimes the chain of causation will run in the other direction: people who suffer from mental illness may be more likely to turn to drugs.
• Sometimes the drug use and the mental ill-health may arise separately and not be related.
• And sometimes the relationship may be even more complex, for example with mental illness encouraging drug use as a form of “self-medication”, which then in turn worsens the illness.

Diminished Responsibility

In a murder case, a further consideration will be the partial defence of diminished responsibility. This article will not attempt a full discussion of every element of the defence, which is found in s.2 of the Homicide Act 1957. However, when considering a defendant with mixed drug use and mental illness, one element of the defence is likely to be particularly contentious – whether any abnormality of mental functioning “arose from a recognised medical condition”.

If the abnormality of mental functioning only arises as a result of the acute effects of voluntary substance abuse, the defence will fail, even though for example “acute intoxication” is a medical condition recognised in diagnostic manuals (R v Dowds). That appears to extend even to a drug induced psychosis which might last for weeks or months after the drug use, if R v Lindo is rightly decided (see especially [8] and [42]).

There may be a narrow path for a defendant who can argue that they suffered from an entrenched addiction such that their substance abuse was no longer voluntary at all – diminished responsibility has arisen in cases of alcoholism (R v Wood), and there would seem no principled reason not to include addiction to other drugs. The defence must first show that the defendant did in fact have such an addiction that meant they could not help taking the substances; and then that the intoxication that resulted caused an abnormality of mental functioning which caused substantial impairment (and so on – see s.2).

However, given that the burden of proof is on the defence, and given that any defendant who has such an entrenched addiction will almost certainly have built up a considerable tolerance to the drug, it may be a rare case where a defence along these lines is likely to succeed.

A more promising defence would be that the defendant had, in the past, abused drugs and had as a result developed ongoing mental ill-health – which continued even though the defendant had now been abstinent for some time. Given Lindo, it seems this would not work if a particular psychotic episode continued throughout, but if the defendant’s condition had abated, but then he had relapsed despite not having taken further drugs, there would seem to be no reason why the defence could not arise.

Of course again, in practice, defendants who have suffered from drug addiction long enough to develop lasting mental illness are more likely to be drawn back to drug use, and it may be that in most cases the defence lawyer will find that their client either does not claim to have remained completely abstinent, or would at least struggle to prove that they had.

Mitigation

Often, the defence practitioner will find themselves here. Defences have been ruled out or have been tried and failed, and the question becomes to what extent the defendant’s mental illness provides mitigation.

Many of the same issues arise. If the mental illness arose independently of the drug use; or if it was caused by drug use but has endured despite the defendant now being abstinent, then the position will be stronger. Most judges are likely to give more credence to such an account if there is evidence of the defendant’s recent abstinence. Considerations in the Sentencing Council’s guideline on “Sentencing offenders with mental disorders, developmental disorders, or neurological impairments” will still arise, particularly the extent to which that mental ill-health actually caused or contributed to the offending.

On the other hand, if any mental ill-health at the time of the offence cannot be divorced from the defendant’s voluntary drug use, it’s likely to provide little mitigation, and certainly not where it is an acute state caused by recent drug use (R v Richfield).

Allison Summers K.C. & Giles Fleming recently represented a defendant charged with murder for whom many of these questions arose. The defendant, H, was charged in what was originally a “missing body” case, but ultimately pleaded guilty to murder and revealed the location of the remains of the deceased. Despite the significant aggravation of the concealment of the body and extensive dismemberment, mitigation led to a minimum term of just over 21 years, before deduction of time served on remand.

Allison & Giles were instructed by Alison Muir of MCP Solicitors.

About the Barristers

Allison Summers KC was called to the bar in 2000 and took Silk in 2020. She is ranked as a Leading Silk in Chambers UK Bar 2026 and Legal500 2026. Allison has a national defence-based practice, and specialises in cases involving homicide, other serious violence, and sexual offences. She appeared in the 2022 BBC documentary Sexsomnia: Case Closed? which looked at some of the legal issues relating to prosecuting and defending cases when sexsomnia was raised.

To instruct Allison, please contact the Practice Director, Chris Feathers, on 020 7404 1881

Giles Fleming was called to the bar in 2016. He has a busy practice both prosecuting and defending across a wide range of offences, including offences of murder, organised crime, modern slavery, and white-collar crime. He is a CPS Grade 3 prosecutor and is appointed to the SFO C panel.”

To instruct Giles, contact his clerks Steve McCarthy or Amie Harris on 020 7404 1881.